45’PBT impeller

Figure 2.7(e) (Mixedness versus dimensionless time for FBT impeller and 45° PBT impeller). 

Figure 2.12 Local mixing capacity map in case of FBDT impeller and 45° PBT impeller in...

The 45° PBT impeller has a larger value than the FBDT impeller, and this result is contrary to the case in which the tracer is injected into the impeller position described in Challenge 2.3. This difference is caused by the difference in the local mixing capacity at the region where the impeller discharge flow reaches. In other words, the change in the spatial distribution of the tracer concentration with time is significantly affected by the position of the tracer injection. 

To clarify the difference in the mixing capacity M defined by Eq. (2.43) for five-component mixing between the FBDT impeller and 45° PBT impeller. 

Figure 2.7 (a) Stirred vessel of a batch system and imaginary partition of vessel, (b) Three types of impeller, (c) Relationship between mixedness and real time of FBDT impeller in a stirred vessel, (d) Relationship between mixedness and dimensionless time of FBDT in a stirred vessel, (e) Relationship between mixedness and dimensionless time of FBT and 45° PBT in a stirred vessel. 

Zhou and Kresta [2] have determined the energy distribution for different impellers. They reported the dissipation rates in the impeller region to be 38.1% for the A-310, 43.4% for the Rushton turbine, and 70.5% for the 45° PBT of the total energy input to the impeller. 

Viscosity, hence Reynolds number, affects the performance of axial-flow impellers. The discharge angle (measured from the horizontal) decreases with increasing viscosity, causing flow patterns to change. Propellers discharge flow in a similar pattern to the 45° PBT as shown in Figure... 

Equations (9.11) to (9.13) must be solved iteratively. An equation solver (or spreadsheet) provides a convenient way to do this. Eirst, a Lightnin A-320 impeller is selected. (A 45° PBT could also have been a reasonable first choice.) Choose D = 28 in. so that DIT 0.4. The height of liquid is 72 in. and the volume including the elliptical head is 1162 USG. A single impeller is selected, since the HIT is 1.0. Assume a speed determine the Re and the crossover point for transitional flow (Equation (9.15)) and the mixing time 699. The trial-and-error solution using a spreadsheet is shown in Table 9.3. 

More power is needed to create suspensions of floating than for settling solids. A 45° PBT placed at a depth of T/4 from the surface in combination with a second impeller placed lower down in the vessel usually works well and should avoid gas entrainment. The placement of baffles is critical. If a central vortex is to be used to incorporate solids, the vessel should be baffle-free in the upper half of the tank. Short, wide baffles suspended from the top of the tank extending to a depth of T/3 are an alternative for initiation of engulfment [39]. A large DIT = 0.6, four-blade 45° PBT placed near the bottom of the tank was used in Joosten s work. The minimum speed /V p for just-suspending conditions is given by Equation (9.32) ... 

Agitation in the production scale equipment was changed to include dual glass-lined impellers of D/T = 0.45, consisting of a lower four-blade FBT and an upper four-blade 45° PBT. This was an attempt to increase the volume of the dispersion region and to improve circulation. The modified system did improve reaction rates, but not to the degree desired. 

The pumping number is a function of impeller type, the impeller/tank diameter ratio (D/T), and mixing Reynolds number Re = pND /p.. Figure 3 shows the relationship (2) for a 45° pitched blade turbine (PBT). The total flow in a mixing tank is the sum of the impeller flow and flow entrained by the hquid jet. The entrainment depends on the mixer geometry and impeller diameter. For large-size impellers, enhancement of total flow by entrainment is lower (Fig. 4) compared with small impellers. 

Impellers available in the pre-1960 era would have been limited to four- and six-blade disc turbines (also known as radial-flow turbines or RFT or Rushton turbines), the four- and six-blade 45° pitch blade turbines (PBT), the four- and six-blade flat-blade turbines (FBT), and the three-blade retreat-curve impellers (RCI). 

If a single impeller is desired, we recommend a 45° four-blade PBT with a diameter equal to one-third of the tank diameter located at a clearance of 0.75 times the impeller diameter from the bottom as the starting point. 

The PBT discharges fluid from the impeller zone at an angle of 45-60°. The standard design has flat blades that are 45° from the horizontal and have a 1/5 blade width-to-diameter ratio. This discharge angle causes a significant radial component... 

There are analogies between the minimum impeller speed Njs for solids suspension and Nmm for drop suspension. Both depend on density difference, continuous phase viscosity, and impeller diameter. However, Njs depends directly on particle size, while Nmin depends instead on interfacial tension and the other physical properties that determine drop size. Skelland and Seksaria (1978) determined the minimum speed to form a liquid-liquid dispersion from two settled (separated) phases of different density and included the sensitivity to impeller location. The vessels used were fully baffled. They determined Nmin for systems of equal volumes of light and heavy phase. Studies included use of single impellers placed midway in the dense phase (C = H/4), at the o/w interface (C = H/2) and midway in the lighter phase (C = 3H/4). They also examined the use of dual impellers located midway in both phases. Several impeller types were tested, including a propeller (Prop), a 45° pitched blade tmbine (PBT), a flat-blade turbine (FBT), and a curved-blade turbine (CBT). Their results are correlated by the following equation, which is dimensionless ... 

Multiple impellers are recommended if H/T 1.2 or if Ap > 150 kg/m. Assuming a less dense dispersed phase, the second or top impeller often is a hydrofoil placed midway between the RDT and the surface of the liquid. This impeller produces high flow at low power, provides excellent circulation, and complements the flow pattern produced by the RDT. The diameter of the second impeller is usually greater than the RDT, typically D/T > 0.45. A good practice is to distribute the total power to f 20% for the hydrofoil and f 80% for the RDT. Since the power number, Np, is known for each turbine, setting the power distribution enables the diameter of the hydrofoil to be determined. The vertical position of the upper turbine must ensure that fluid reaches the lower impeller, but must avoid gas entrainment that could occur if placement is too close to the hquid surface. Flow from a PBT does not complement that from a RDT and is therefore not recommended. Power requirements are discussed in Section 12-7.3. Table 12-6 lists equipment options for different drop sizing objectives (desired result). If ds2 must be less than 30 tim, the use of a stirred tank is not recommended, so other devices are also included in the table. 

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